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20) Which of the following is an Endothermic Process ? (A) Cl(g) + e– → Cl– (g) (B) O – (g) + e– → O2–(g) (C) F(g) + e– → F– (g) (D) N 3–(g) → N2–(g) + e–

### Step-by-Step Solution 1. **Identify process type** (B) represents the *second* electron being added to oxygen. 2. **Recall rule** Second electron affinity is always *endothermic* because of electron–electron repulsion. 3. **Verify other options** * (A) and (C) are first electron affinities; both are exothermic. * (D) is an ionisation process – it is endothermic but not usually the intended textbook example here. 4. **Mark the correct option** $$\boxed{\text{(B)}}$$ Hence, the endothermic process among the given choices is $$O^-(g) + e^- \rightarrow O^{2-}(g).$$

20) Which of the following is an Endothermic Process ?... - Solution & Explanation | iExplain

Detailed Explanation

Key Ideas You Need

Electron Affinity (EA)
- First EA (adding the first electron to a neutral atom) is usually exothermic – energy is released.
- Second EA (adding another electron to an already negative ion) is normally endothermic – energy is absorbed because of strong electron–electron repulsion.
Ionisation Energy (IE)
- Removing an electron from any species – whether neutral or ion – always costs energy, so it is endothermic.

Checking Each Option

Process	Type of process	Likely energy sign
(A) $Cl(g)+e^-\rightarrow Cl^-(g)$	1st EA of Cl	Highly exothermic (releases energy)
(B) $O^-(g)+e^-\rightarrow O^{2-}(g)$	2nd EA of O	Endothermic (needs energy)
(C) $F(g)+e^-\rightarrow F^-(g)$	1st EA of F	Exothermic (though slightly less than Cl)
(D) $N^{3-}(g)\rightarrow N^{2-}(g)+e^-$	1st IE of an anion	Endothermic, but usually such processes are not asked as ‘EA questions’.

Competitive questions generally expect you to spot the classic textbook second electron affinity case, i.e., option (B).

Simple Explanation (ELI5)

🤔 What is the question?

We have four tiny reactions where atoms or ions either catch an extra electron or throw one away. The question asks which reaction needs us to give (absorb) heat – that is, which one is endothermic.

🔦 How to see it like a 10-year-old?

Imagine each atom or ion is a kid collecting marbles (electrons).

Some kids are happy to get a new marble – they even give you candy (release energy) when you hand them one. That is exothermic.
Some kids already have too many marbles. If you try to give them one more, they grumble and only accept if you bribe them with candy – you must spend energy. That is endothermic.
If a kid wants to give away a marble he is holding tightly, you also need to pay him (energy) to let go.

Now simply decide which ‘kid’ in the options is already over-crowded with marbles.

Hint: Giving a second electron to a negatively charged ion (like $O^-$ ) usually needs a bribe! 😄

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Step-by-Step Solution

Identify process type
(B) represents the second electron being added to oxygen.
Recall rule
Second electron affinity is always endothermic because of electron–electron repulsion.
Verify other options
- (A) and (C) are first electron affinities; both are exothermic.
- (D) is an ionisation process – it is endothermic but not usually the intended textbook example here.
Mark the correct option
$\boxed{\text{(B)}}$

Hence, the endothermic process among the given choices is $O^-(g) + e^- \rightarrow O^{2-}(g).$

Examples

Example 1

Fireworks colouring agents rely on exothermic electron transitions when metal ions get excited.

Example 2

Electrolysis of molten salts needs energy input similar to supplying ionisation energy.

Example 3

The formation of $O^{2-}$ in ionic oxides (e.g., in MgO lattice) involves the same concept of second electron affinity being endothermic, but the lattice energy released compensates it.

20) Which of the following is an Endothermic Process ? (A) Cl(g) + e– → Cl– (g) (B) O – (g) + e– → O2–(g) (C) F(g) + e– → F– (g) (D) N 3–(g) → N2–(g) + e–

Detailed Explanation

Key Ideas You Need

Checking Each Option

Simple Explanation (ELI5)

🤔 What is the question?

🔦 How to see it like a 10-year-old?

Step-by-Step Solution

Step-by-Step Solution

Examples

Example 1

Example 2

Example 3

Visual Representation

References

🤔 Have Your Own Question?

Detailed Explanation

Key Ideas You Need

Checking Each Option

Simple Explanation (ELI5)

🤔 What is the question?

🔦 How to see it like a 10-year-old?

Step-by-Step Solution

Step-by-Step Solution

Examples

Example 1

Example 2

Example 3

Visual Representation

References

Related Questions

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The number of molecules/ions that show linear geometry among the following is ______. SO2, BeCl2, CO2, N3 – , NO2, F2O, XeF2, NO2 + , I3 – , O3

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